As micronization of a semiconductor element advances, the type of a light source used for a semiconductor exposure apparatus is being shifted from the g-line of a high-pressure mercury-vapor lamp to an i-line having a shorter wavelength and a laser beam source (KrF or ArF), called an excimer laser, which emits light having a much shorter wavelength. In recent years, the use of an X-ray beam is also proposed. Moreover, realizing higher resolving power and larger depth using a phase shift mask or a modified illumination has been proposed and even begun to be practiced. However, the method using the excimer laser or X-ray beam increases the cost of the apparatus, and the phase shift mask or modified illumination is not effective for some circuit patterns.
On the other hand, increasing the NA (numerical aperture) of a projection lens makes it possible to realize a higher resolving power.
To cope with this trend, the practical application of a liquid immersion method of supplying a liquid between the projection lens and the imaging plane to increase its NA using the refraction effect is being attempted. For example, Japanese Patent Publication No. 63-49893 discloses a liquid immersion projection exposure apparatus, which forms a nozzle having a liquid supply port, so as to surround the distal end of a reduction lens, supplies a liquid via the nozzle, and holds the liquid between the reduction lens and the wafer.
As for exposure amount control, to measure a light amount intensity difference between apparatuses and to guarantee an equivalent exposure amount among a plurality of apparatuses, an illuminometer (external illuminometer) is externally loaded in the apparatus and an exposure amount sensor (internal illuminometer) in the apparatus is calibrated. Furthermore, to convert a value output from the internal illuminometer into an absolute light amount value, the internal illuminometer must be calibrated by an absolute light amount sensor. In fact, it is inefficient to separately execute absolute light amount calibration and calibration between the apparatuses. Therefore, these two calibrations are executed using the same absolute light amount sensor as the external illuminometer.
In the above conventional liquid immersion projection exposure apparatus, the transmittance of an excimer laser beam changes between a dried state and a liquid immersion state. In the dried state, the space between the reduction lens and the wafer is filled with a gas, such as air. In the liquid immersion state, the space between the reduction lens and the wafer is filled with a liquid to attain micropatterning. As a result, the illuminance of the imaging plane changes between these states. Furthermore, when the external illuminometer is set in the liquid immersion state, downtime is prolonged due to illuminance measurement and close attention must be paid to prevent the liquid from leaking outside a region where liquid immersion is possible, resulting in poor working efficiency. This makes it difficult to assure a good exposure amount control performance in the liquid immersion state, as with the conventional dried state.